This invention relates to ferroelectric compositions, to ferroelectric vapor deposition targets and to methods of making ferroelectric vapor deposition targets.
Ferroelectric materials are a family of high K dielectric materials that are starting to become used more in the microelectronics fabrication industry. In addition to having high dielectric constants, ferroelectric materials typically have low leakage current and non-volatile data retention properties which make them attractive as dielectric materials for memory and transistor devices. Ferroelectric materials exhibit a number of unique and interesting properties. One such property of a ferroelectric material is that it possesses a spontaneous polarization that can be reversed by an applied electric field. Specifically, these materials have a characteristic temperature commonly referred to as the transition temperature, at which the material makes a structural phase change from a polar phase (ferroelectric) to a non-polar phase, typically called the paraelectric phase. Example ferroelectric materials include titanates and tantalates, such as by way of example only, lead lanthanum zirconium titanate (PLZT), barium strontium titanate (BST), and strontium bismuth tantalate (SBT).
As memory cell density and other circuitry density increases, there is a continuing challenge to maintain sufficiently high storage capacitance in capacitors despite the decreasing size. Additionally, there is a continuing goal to further decrease capacitor size. One principal way of increasing cell capacitance is through cell structure techniques. Yet as feature size continues to become smaller and smaller, development of improved materials for the cell dielectric become increasingly important. Conventional non-ferroelectric dielectric materials, such as SiO2 and Si3N4, are not expected to be suitable in most applications where device dimensions decrease to 0.25 micron in width because of the expected requirement for a very thin dielectric film. This is expected to apply in most all thin film dielectric material applications.
In addition to use as transistor and capacitor dielectrics, ferroelectric materials might also be used in microelectronic mechanical systems. These devices are mechanical three-dimensional constructions with sizes in the micrometer ranges. Sensors and actuators are example two main categories of microelectronic mechanical systems. Ferroelectric thin films have been proposed for use with silicon-based microelectronic mechanical systems for both sensors and actuators in a variety of applications.
Thin film ferroelectric materials are known within the art to be deposited by chemical vapor deposition, chemical solution deposition or physical vapor deposition. Physical vapor deposition includes sputtering, laser ablation, and other existing and to-be-developed methods. Existing ferroelectric physical deposition targets are typically made using conventional powder metallurgy with either cold press sintering or hot pressing. Such prior methods can include the provision of prereacted ferroelectric powders having individual particles sized at greater than or equal to 1 micron. In hot pressing, such powders are consolidated at high temperatures and pressure, and typically results in targets having non-uniform grain sizes of from 1 micron to 50 microns and non-uniform microstructure comprising multiple phases.
It would be desirable to improve upon existing ferroelectric physical vapor deposition targets and their methods of manufacture.
The invention comprises ferroelectric compositions, ferroelectric vapor deposition targets and methods of making ferroelectric vapor deposition targets. In one implementation, a ferroelectric physical vapor deposition target has a predominate grain size of less than or equal to 1.0 micron, and has a density of at least 95% of maximum theoretical density.
In one implementation, a method of making a ferroelectric physical vapor deposition target includes positioning a prereacted ferroelectric powder within a hot press cavity. The prereacted ferroelectric powder predominately includes individual prereacted ferroelectric particles having a maximum straight linear dimension of less than or equal to about 100 nanometers. The prereacted ferroelectric powder is hot pressed within the cavity into a physical vapor deposition target of desired shape having a density of at least about 95% of maximum theoretical density and a predominate maximum grain size 5 which is less than or equal to 1.0 micron.
In one implementation, the prereacted ferroelectric powder is hot pressed within the cavity into a physical vapor deposition target of desired shape at a maximum pressing temperature which is at least 200xc2x0 C. lower than would be required to produce a target of a first density of at least 85% of maximum theoretical density in hot pressing the same powder but having a predominate particle size maximum straight linear dimension of at least 1.0 micron at the same pressure and for the same amount of time, and a target density greater than the first density at the lower pressing temperature is achieved.